1. Field
The present disclosure relates generally to a method for transferring biomass into a reactor vessel to produce a liquefied fuel product. In particular, the present disclosure relates to a method for transferring a pressurized slurry of biomass into a high pressure, high temperature reactor vessel to produce a bio-oil product that can be converted into biofuels.
2. Related Art
Biomass offers a potentially renewable source for fuel, supplementing or replacing petroleum, coal and natural gas. Biomass typically comprises large amounts of cellulose, often bound together by lignin. Cellulose and lignin can be converted into biofuels via thermochemical conversion processes.
Many processes are known for converting a slurry of biomass and liquid into liquid fuels. These processes may include hydrothermal pyrolysis followed by hydroprocessing, supercritical gasification or liquefaction followed by Fischer-Tropsch or other synthesis, high temperature hydrolysis catalyzed with an acid or base followed by fermentation or catalytic reforming, and solvent liquefaction followed by hydroprocessing.
Of these processes, solvent liquefaction is performed at moderate pressure and temperature. For example, early efforts involved hydrogenation with hydrogen gas at high temperatures to convert lignin-containing biomass into a liquid fuel. More recent efforts involve solvent liquefaction processes, typically conducted at high temperatures (e.g., at least 200° C.) and high pressures (e.g., at least 200 psi). The solvent liquefaction process typically employs a hydrogen donor solvent to reduce the oxygen content of the biomass, which in turn increases the energy content of the biomass for use as combustible fuels. Liquefaction produces a liquid product that usually still has an oxygen content that is above the specifications needed for refinery blendstocks. Accordingly, the liquid product will generally undergo an additional step of hydroprocessing for conversion into useful refinery blendstock products.
Biomass is typically harvested and delivered in varying sizes. For many of these known processes performed at high pressures and/or high temperatures, the physical size of the biomass needs to first be reduced. Methods known in the art to reduce biomass size include chipping, grinding, shredding, chopping, and milling.
Reducing biomass size helps improve mass transfer or chemical reaction kinetic (reaction) rates, but requires capital equipment and energy inputs that can exceed the benefit. Generally, the biomass size specification is a compromise that is optimized when the incremental cost of more size reduction is equal to the incremental benefit of faster reactions resulting. However, in the case of high pressure reactions, the feeding system often dictates the size reduction required.
Currently, feeding more practical-sized pieces of biomass into a high pressure, high temperature reactor vessel at commercial scale is expensive and energy inefficient using conventional equipment. Plug screw devices, high pressure rotating feeders, and rotating star type feeders may be used to feed slurries of biomass and liquid into reactor vessels; however, the limit in the feeding device in industrial/commercial use is about 150-200 psi due to mechanical seal limits. Other types of equipment such as extruders may be used, but these have not been used with biomass at high throughputs, have relatively high capital costs, and have relatively high electricity demands. While lock hopper designs are technically feasible, they do not support continuous operation, which confers a commercial advantage for scaling up reactions compared to batch operations. Moreover, lock hopper designs present significant problems with respect to keeping the seals clean and free of reaction materials. Reciprocating pumps may also be used, but require materials with very fine particle size, making this type of equipment commercially unattractive to convert biomass into liquid fuels.
What is needed in the art is a commercially viable method that is energy efficient, reliable, and continuous for feeding practical-sized biomass into a high pressure, high temperature reactor. What is also needed is a commercially viable method that can vary the ratio of biomass to liquid that enters the reactor. In particular, a method to feed a pressurized slurry of biomass into a high pressure, high temperature reactor vessel, without needing to grind the biomass is desired. Once the pressurized slurry of biomass is fed into the reactor vessel, the biomass can be converted into a gas, liquid fuel, and/or a bio-oil product that may be used to produce fuels or chemicals.